1. Field of the Invention
This invention relates to a process and apparatus for extracting liquid from organic and/or inorganic sludge to form a sludge cake, i.e., an end product that is a solid or semi-solid and thus lacks the physical characteristic of a liquid sludge. A sludge cake as used herein is defined as the end product in cake, solid or semi-solid form derived by dewatering sludge and generally having a water content less than 90%.
2. Description of the Prior Art
The removal of water from sludge, both organic and/or inorganic, in the waste water treatment field has been a serious problem for a number of years. A number of arrangements have been used, such as press and vacuum filters, capillary band pass filters, centrifuges, thickeners, screening machines and combinations thereof, all of which suffer from a number of serious drawbacks. For example, there is a tendency for blockage of the filter media and rapid heavy wear of the machine parts. In addition, the generally low capacity of a given piece of equipment results in high installation and operating costs. Some of these problems have been minimized by use of an apparatus having a continuously rotating capillary band pass filter effecting a further dehydration by means of pressure rollers but, here also, the efficiencies have fallen short of expectations. pg,4
Several devices for dewatering sludge in horizontal press fashion are known to the art in a number of different constructions. Yet, they have not proven themselves to be completely satisfactory in operation because their practical construction is associated with a series of problems which up to now have not been resolved. One such problem resides in the fact that the forces occurring at the band filter are difficult to control.
When squeezing or pressing products in band filter presses, the press product initially is subjected, as a general rule, to its hydrostatic pressure. In so doing, a greater and greater amount of solid constituents deposit on the filter band. As soon as the solid coating increases, there begins the actual pressing or squeezing operation. While it is desirable that the pressure increase in the filter press climb linearly in accordance with the increasing hydrostatic pressure, the pressure in prior art presses increases very markedly towards infinity. Furthermore, local irregular deposition of solids at the filter press bands produce uncontrolled pressure peaks. Pressure increases in the press zone, as well as the occurrence of pressure peaks considerably change from one type of slurry to another. Additionally, the forces which prevail still are dependent upon certain equipment parameters, such as velocity of movement of the band, filter efficiency, and so forth. For example, there is known to the art a band filter press of the previously mentioned type wherein two support frames arranged in the machine housing serve to support the filter press bands. These filter press bands are situated upon a rigid support with at least one of the supports being pivotally mounted at the infeed side. Both frames are coupled with one another through springs or other resilient elements. The spring elements render it possible to absorb the press pressure in its entirety except for local pressure peaks acting upon the rigid frames.
With the buildup of the thickness of the sludge cake, the pressure exerted between the two belts may not be constant. Further, where the dewatering is dependent entirely on a pressing action, insufficient dewatering may result unless an excessively long belt or very high pressures are utilized.